Biochemical quality of crop residues and carbon and nitrogen mineralization kinetics under nonlimiting nitrogen conditions

Statistical relationships were established between the fate of C and N from 47 types of crop residues and their biochemical characteristics during a s oil incubation at 15 degrees C. The incubations were carried out under nonl imiting N in order to differentiate the effects of biochemical characterist ics of residues from those of soil N availability. Depending on the residue , the apparent mineralization of residue C after 168 d varied from 330 to 6 70 g kg(-1) of added C Mineralization kinetics were described using a two-c ompartment decomposition model that decomposes according to first-order kin etics. Amounts of C mineralized after 7 d and the decomposition rate coeffi cient of the labile fraction were related mainly to the soluble C forms of the residue, No statistical relationship was established between the N conc entration of residues and their decomposition in the soil. The incorporatio n of crop residues into soil led to various soil mineral N dynamics. Two re sidues caused net N mineralization from the time of their incorporation, wh ereas all the others induced net N immobilization (1-33 g N kg(-1) of added C). After 168 d, only residues with a C/N ratio <24 induced a surplus of m ineral N compared with the control soil. The mineral N dynamics were relate d mainly to the organic N concentration of the residues and to their C/N ra tio. At the start of incubation, these dynamics were also influenced by the presence of polyphenols in the plant tissues. Finally, this study show-ed the need to include the biochemical quality of crop residues in any C and N transformation models that describe decomposition. In contrast, the N conc entration or C/N ratio of the residues are sufficient to predict the net ef fects of crop residues on soil mineral N dynamics.